Method and apparatus for improving steam turbine control

ABSTRACT

Steam turbine control systems often incorporate electromechanical pilot-valve actuator assemblies to regulate hydraulically-driven actuator pistons that modulate steam valves, thereby controlling turbine speed. The general operation and quick-response requirements of these pilot-valve actuator assemblies can suffer from shortcomings (such as, excessive friction, contaminated oil, and defective electromagnetic components) that degrade the control system&#39;s overall performance. For these reasons, this disclosure relates to a method for overcoming insufficient, pilot-valve actuator response by using an integrated control system dedicated to these electromechanical assemblies. Pilot-valve actuators can be equipped with a single coil or multiple coils (usually a primary and a secondary). In a dual-coil configuration, the primary is energized according to an output of a PID controller, whereas the secondary coil is regulated by a separate control element. When the speed at which the steam-valve actuator reacts is adequate, the energizing signals to the individual coils may be equal, or the secondary coil may not be energized at all. But whenever the steam-valve actuator&#39;s speed is not adequate, the secondary coil is quickly energized to a higher level; consequently assuring prompt and reliable response of the integrated, turbomachinery speed-control system.

TECHNICAL FIELD

[0001] This invention relates generally to a method and apparatus for speed control of steam turbines. More specifically, the invention relates to a method for overcoming performance degradation attributed to a worn or defective electromechanical pilot-valve actuator assembly (a major component of the overall control system) by employing one or more additional digital controllers; consequently improving the accuracy, stability, and reliability of an integrated, turbomachinery speed-control system.

BACKGROUND ART

[0002] Highly-reliable, generator speed-control is required nowadays to meet the stringent requirements for utility-grid frequency; furthermore, recurrent changes in energy-demand dictate prompt, control-system response. If control-system response is not sufficient during transients, a discrepancy will exist between a generator's rotational speed and the generator speed required to match the utility-grid frequency, in spite of the control system's steady-state accuracy.

[0003] Typically, a steam turbine control-system is equipped with an electromechanical pilot-valve actuator assembly regulated by at least one controller. This actuator drives a pilot valve used to manipulate a hydraulic steam-valve actuator that, in turn, modulates a steam valve, thereby controlling a turbine's speed. At times, however, this particular control setup cannot fully satisfy quick-response requirements because of insufficient electromagnetic force needed to overcome inherent frictional forces and/or other restricting effects that can impede the pilot valve's linear motion. Accordingly, there is a need for a method of control that compensates for performance degradation of the overall integrated system brought about by insufficient pilot-valve response.

DISCLOSURE OF THE INVENTION

[0004] A purpose of this invention is to provide a method for controlling the rate of steam flow through a turbine by monitoring the position of a pilot valve, in addition to monitoring the dynamics of a steam valve; then applying these data to compensate for the deficient action of an electromechanical pilot-valve actuator assembly.

[0005] The pilot valve manipulates a hydraulically-driven, steam-valve actuator that modulates the steam valve through which steam passes into the turbine. Performance degradation can occur when a pilot-valve actuator assembly's operation is faulty due to impaired electromagnetic components, excessive friction, or contaminated oil.

[0006] The aforementioned purpose is accomplished, in part, by employing a unique control system dedicated to pilot-valve actuators which are equipped with a mix of individually-energized induction coils: a single coil or multiple coils (usually a primary and a secondary) whose respective control setups utilize the steam-valve actuator's piston velocity as a process variable.

[0007] Using a dual-coil configuration as an example, a speed-controller Proportional Integral Derivative (PID) algorithm produces an output value, based on the steam turbine's measured rotational-speed and a rotational-speed set point. This PID output value is compared with an actuator position, and the difference between them is then multiplied by a constant gain as a set point for a steam-valve velocity PID controller that uses the previously mentioned steam-valve actuator's piston velocity as its process variable.

[0008] Next, a pilot-valve PID position controller is cascaded with the steam-valve velocity PID controller whose output is used as the pilot-valve position controller's set point. Subsequently, the pilot-valve position controller's output becomes the energizing signal for the pilot-valve actuator's primary coil, whereas the secondary coil is regulated by a separate Proportional Derivative (PD) control element.

[0009] Finally, when the speed at which the steam-valve actuator responds is adequate, both coils are energized proportionately, or one coil may not be energized at all. But whenever the steam-valve actuator's response speed is not adequate, the energizing signal to the secondary coil (and possibly to the primary coil, as well) is quickly increased by a value corresponding to the difference between the required velocity of the steam-valve actuator's piston and the piston's actual velocity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 shows an integrated, turbomachinery speed-control system with a pilot-valve actuator assembly comprising two electromagnetic coils.

[0011]FIG. 2 shows an integrated, turbomachinery speed-control system with a pilot-valve actuator assembly comprising a single electromagnetic coil.

[0012]FIG. 3 shows an integrated, turbomachinery speed-control system with a pilot-valve actuator assembly comprising two electromagnetic coils energized by separate constant-multipliers.

BEST MODE FOR CARRYING OUT THE INVENTION

[0013] To maintain reliable, accurate, and stable speed-control of a constituent steam turbine, an integrated turbomachinery control system should be capable of compensating for the possibility of faulty operation of an electromechanical pilot-valve actuator assembly by monitoring and controlling the position of a pilot valve, as well as assessing the velocity of a steam-valve actuator's piston.

[0014]FIG. 1 shows a steam turbine 101 complete with its speed-control system incorporating a rotational-speed PID controller (#1) 102 that monitors a speed set point (SP) 103, in addition to rotational-speed measurements obtained by a speed transmitter (N) 104. This #1 controller 102 inputs (X_(SP)) to a #1 summation block 105 that receives an additional signal from a transmitter (XMTR 1) 106 monitoring the position (X) of a steam-valve actuator's 107 piston. The steam-valve actuator is connected to a steam valve 108 used to regulate the flow of steam passing through the turbine 101. When steam exits the turbine, it passes into a condenser 109 or other process; additionally, the turbine is used to drive a load 110 (shown as a generator), but this invention is not restricted to a particular load.

[0015] The #1 summation block's 105 calculated value (ΔX) is directed to a logic module 111 [by way of a constant multiplier (K₁) 112 as a velocity set point, V_(SP) In 1] and also to a steam-valve velocity PID controller (#2) 113 [by way of a second, constant multiplier (K₂) 114]; note that K₁<K₂.

[0016] In addition to inputting to #1 summation block 105, XMTR 1 106 also inputs to a time-derivative function block (d/dt) 115 that calculates the steam-valve actuator's piston velocity (V) from the measured values of the piston's position (X). This velocity value (V In 2) is then allocated to the logic module 111 and to controller #2 113 whose output is directed to a third PID controller (#3) 116. Controller #3 monitors the position of a pilot valve 117 [by way of a second transmitter (XMTR 2) 118]; this #3 controller's output is connected to the primary coil 119 of an electromechanical actuator (ACTR) 120 that drives the pilot valve 117 which, by way of hydraulic fluid, activates the steam-valve actuator 107 causing a change in its piston's position (X). XMTR 2 118 also sends a pilot-valve position signal to the logic module (In 3) 111.

[0017] In its illustrated configuration, the turbine-generator set participates in control of the turbine's rotational speed (which is proportional to the generator's 110 rotational speed). When changes in the turbine's rotational speed occur, the speed transmitter's 104 output signal (N) will vary which, in turn, results in a modified signal from PID controller #1 102 to the #1 summation block 105. While in a steady-state (equilibrium) condition, #1 summation block's output is zero; however, a nonzero output (augmented by K₂ 114) is the velocity set point for PID controller #2 113. This velocity set point is for the steam-valve actuator 107, and when compared with a velocity value (V) 115 it is transformed (through the PID algorithm) by controller #2 113 into a position set point for the pilot valve 117 whose response directly energizes the primary coil 119. Subsequently, a feedback value for the pilot valve's 117 position (by way of the electromechanical actuator 120) is transmitted from XMTR 2 118 as the process variable for controller #3 116.

[0018] When the turbine's rotational speed (N) changes, the control system responds by ultimately changing the signals to at least the primary coil of the pilot-valve actuator 120. This action, in turn, initiates a modulation of the pilot valve 117, causing a change in the position of the steam-valve actuator's 107 piston. Accordingly, the steam valve 108 assumes a new position corresponding to the required control-system response. A change in the steam-valve actuator's 107 piston position translates to a change in the turbine's rotational speed. Simultaneously, the three inputs (V_(SP) In 1, V In 2, and In 3) to the logic module 111 are used to calculate two outputs as follows: Condition  1: $\quad {{V_{SP}} < {{V}\left\{ {{\begin{matrix} {\text{Out~~1} = 0} \\ {\text{Out~~2} = 0} \end{matrix}\text{Condition~~2:}\quad {V}} \leq {{V_{SP}}\left\{ \begin{matrix} {\text{Out~~1} = {V_{SP} - V}} \\ {\text{Out~~2} = {{\ln \quad 3} - {\ln \quad 3_{m}}}} \end{matrix} \right.}} \right.}}$

[0019] where

[0020] V_(SP)=K₁ΔX

[0021] ΔX=steam-valve actuator's piston-position deviation (X_(SP)−X)

[0022] V=dX/dt

[0023] In 3=pilot-valve position signal (inputted to logic module 111)

[0024] In 3_(m)=In 3 signal stored in memory when Condition 2 first becomes true. This value remains constant until Condition 2, for which this value is being used, is no longer true.

[0025] Conditions 1 and 2, as shown, assume that the signs of V and V_(SP) are the same.

[0026] Out 1 is used directly as a set point for controller #4 121 [shown utilizing a Proportional Derivative (PD) algorithm]. Out 2 is passed on to controller #4 as a process variable.

[0027] In one embodiment of this invention (FIG. 1), the secondary coil 122 is energized by a value proportional to that used to energize the primary coil 119. The secondary coil's energizing signal is inputted from the #2 summation block 124 that combines a constant-of-proportionality (K₃) 123 with the output of controller #4 121. Usually, the value of K₃ is zero, in which case the secondary coil 122 is energized only when Condition 2 is true, or when the value of K₃ is 1.0 so that the secondary coil is energized the same as the primary coil 119. In reality, the value of K₃ 123 can be chosen to be any value producing the desired response from the electromechanical actuator 120.

[0028] As long as Condition 1 is true, the secondary coil 122 is energized to the level “K₃ times the output of controller #3 116” (this product may be zero) because both the process variable and the set point for controller #4 121 are zero. (The derivative term could be nonzero shortly after the process variable and the set points become zero, but it would quickly become zero.)

[0029] If Condition 2 is satisfied, controller #4's 121 set point equals the difference between the steam-valve actuator's piston-velocity set point and the piston's velocity. As soon as Condition 2 becomes initially satisfied, the process variable for controller #4 121 will be zero because In 3_(m) is equal (at that instant) to In 3.

[0030] As time progresses, while Condition 2 is satisfied, In 3 will vary while In 3_(m) remains constant; for this reason, controller #4's 121 process variable will deviate from zero. With additional electromagnetic force, due to the now energized secondary coil 122, the steam-valve actuator's piston velocity will quickly meet or exceed the required velocity (V_(SP)); at which point, Condition 2 will no longer be satisfied.

[0031] A second embodiment (FIG. 2) shows the output signals from PID controller #3 116 and PD controller #4 121 being summed in block #2 124: the single coil 201 is energized, based on the summation block's signal. All other aspects of the control scheme are the same.

[0032]FIG. 3 displays a third embodiment employing multiple coils 119, 122 in which the signals used to energize the respective coils are multiplied by factors K₃ 123 and K₄ 301. Here, the two coils are energized proportionally, each contributing to the electromagnetic force required to activate the electromechanical pilot-valve actuator 120, regardless of which Condition (1 or 2) is in effect. All other aspects of the control scheme are the same.

[0033] The turbine-controlled variable described herein is not restrictive nor unique to this invention; in which case, other control-system variables may be considered. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. 

We claim:
 1. A method for controlling a steam turbine system comprising a steam turbine, a hydraulic steam-valve actuator, instrumentation providing a signal proportional to a steam-valve actuator's piston velocity, a pilot valve for manipulating a position of the steam-valve actuator's piston, and an electromechanical pilot-valve actuator containing at least one individually-energized coil, the method comprising: (a) sensing the steam-valve actuator's piston-velocity signal; and (b) sending a signal to at least one individually-energized coil within the pilot-valve actuator, a strength of the signal being based on an error between the sensed piston velocity and a set point for the piston velocity.
 2. The method of claim 1, wherein the set point for the steam-valve actuator's piston velocity is calculated, based on an error between a steam turbine controlled-variable and a set point.
 3. The method of claim 2, wherein the steam turbine controlled-variable is a steam turbine rotational-speed.
 4. The method of claim 2, wherein the steam turbine controlled-variable is a steam-valve actuator's piston position.
 5. The method of claim 1, wherein the steam turbine system also comprises a control system sensing at least one controllable variable, and controlling the variable to the variable's set point; and at least one coil is energized by a signal, based on an output from the system controlling at least one controllable variable.
 6. The method of claim 5, wherein the controllable variable is the velocity of the steam-valve actuator's piston.
 7. The method of claim 5, wherein the controllable variable's set point is proportional to an error between a steam turbine rotational-speed and a rotational-speed set point.
 8. The method of claim 5, wherein the controllable variable's set point is proportional to an error between a steam-valve actuator's required piston velocity and an actual piston velocity.
 9. The method of claim 5, wherein the controllable variable is an error between a pilot-valve's position and a position at a predetermined point in time.
 10. The method of claim 9, wherein the predetermined point in time is the point at which a magnitude of the steam-valve actuator's piston velocity becomes less than a magnitude of a required piston velocity.
 11. The method of claim 5, wherein at least one coil is not energized whenever a magnitude of the steam-valve actuator's piston velocity is greater than or equal to a magnitude of a required piston velocity.
 12. The method of claim 5, wherein at least one coil is energized to a level determined by the sum of: (a) a value proportional to the error between the sensed piston velocity and a set point for the piston velocity; and (b) a value proportional to an output from the system controlling at least one controllable variable.
 13. The method of claim 12, wherein all coils are energized by values proportional to the sum.
 14. The method of claim 1, wherein the steam-valve actuator's piston-velocity signal is generated by the steps of: (a) sensing a steam-valve actuator's piston position; (b) generating a signal, based on the piston position; and (c) computing a first time-derivative of the piston-position signal.
 15. The method of claim 3, wherein the set point for the steam-valve actuator's piston velocity is calculated by the steps of: (a) calculating a speed controller output, based on the steam turbine rotational-speed and set point; (b) sensing a steam-valve actuator piston position; (c) calculating a difference between the speed controller output and the steam-valve actuator position; and (d) calculating the set point for the steam-valve actuator's piston velocity as being proportional to the difference between the speed controller output and the steam-valve actuator position.
 16. An apparatus for controlling a steam turbine system comprising a steam turbine, a hydraulic steam-valve actuator, instrumentation providing a signal proportional to a steam-valve actuator's piston velocity, a pilot valve for manipulating a position of the steam-valve actuator's piston, and an electromechanical pilot-valve actuator containing at least one individually-energized coil, the apparatus comprising: (a) means for sensing the steam-valve actuator's piston-velocity signal; and (b) means for sending a signal to at least one individually-energized coil within the pilot-valve actuator, a strength of the signal being based on an error between the sensed piston velocity and a set point for the piston velocity.
 17. The apparatus of claim 16 also comprising means for calculating the set point for the steam-valve actuator's piston velocity, based on an error between a steam turbine controlled-variable and a set point.
 18. The apparatus of claim 17, wherein the steam turbine controlled-variable is a steam turbine rotational-speed.
 19. The apparatus of claim 17, wherein the steam turbine controlled-variable is a steam-valve actuator's piston position.
 20. The apparatus of claim 16, wherein the steam turbine system also comprises a control system with means for sensing at least one controllable variable, and means for controlling the variable to the variable's set point; and at least one coil is energized by a signal, based on an output from the system controlling at least one controllable variable.
 21. The apparatus of claim 20, wherein the controllable variable is the velocity of the steam-valve actuator's piston.
 22. The apparatus of claim 20 also comprising means for calculating the controllable variable's set point as proportional to an error between a steam turbine rotational-speed and a rotational-speed set point.
 23. The apparatus of claim 20 also comprising means for calculating the controllable variable's set point as proportional to an error between a steam-valve actuator's required piston velocity and an actual piston velocity.
 24. The apparatus of claim 20 also comprising means for calculating the controllable variable as an error between a pilot-valve's position and a position at a predetermined point in time.
 25. The apparatus of claim 24 comprising means for determining the predetermined point in time as the point at which a magnitude of the steam-valve actuator's piston velocity becomes less than a magnitude of a required piston velocity.
 26. The apparatus of claim 20, wherein at least one coil is not energized whenever a magnitude of the steam-valve actuator's piston velocity is greater than or equal to a magnitude of a required piston velocity.
 27. The apparatus of claim 20 including means for energizing at least one coil to a level determined by the sum of: (a) a value proportional to the error between the sensed piston velocity and a set point for the piston velocity; and (b) a value proportional to an output from the system controlling at least one controllable variable.
 28. The apparatus of claim 27 including means for energizing all coils by values proportional to the sum.
 29. The apparatus of claim 16, wherein the steam-valve actuator's piston-velocity signal is generated, the apparatus comprising: (a) means for sensing a steam-valve actuator's piston position; (b) means for generating a signal, based on the piston position; and (c) means for computing a first time-derivative of the piston-position signal.
 30. The apparatus of claim 18, wherein the set point for the steam-valve actuator's piston velocity is calculated, the apparatus comprising: (a) a speed controller calculating its output, based on the steam turbine rotational-speed and set point; (b) a steam-valve actuator piston-position sensor; (c) a difference calculator for calculating a difference between the speed controller output and the steam-valve actuator position; and (d) a set point calculator for calculating the set point for the steam-valve actuator's piston velocity as being proportional to the difference between the speed controller output and the steam-valve actuator position. 